The fuel needed to obtain specified thrust levels from a turbofan engine varies as a function of the temperature of the operating environment. To properly set fuel schedules, the standard fuel control equipment contains a bellows element. The bellows element responds to the temperature of the air in which it is bathed and then through mechanical linkages sets both inlet guide vane position and fuel flow schedules for the engine.
The task of supplying the sensing bellows with a continuous supply of fresh air has usually been achieved as follows. A small forward facing airscoop is placed in the inlet ducting in front of the forward fan stage. Tubing connects the airscoop to the inlet end of the fuel control equipment. At the outlet of the fuel control equipment another tube leads back to an aft facing discharge nozzle situated immediately in front of the forward fan stage. In theory the entering ram air will be picked up by the scoop, flow through the fuel control equipment and its associated bellows, then be drawn back by the suction of the rotating fan just downstream of the aft facing discharge nozzle. In practice, it has been found that with the system described above, the fuel control unit does not always get a supply of fresh ambient air. At certain flight speeds or under some crosswind conditions, standing waves or pressure nulls will build up in the vicinity of the airscoop and the aft facing nozzle. This condition can bring about stagnation of air flow at the sensing bellows under these flight regimes. The result is an erroneous fuel control setting. My invention corrects this problem by keeping a fresh supply of ambient air flowing past the bellows at all times.
The related art shows a broad range of fuel control equipment. U.S. Pat. No. 2,857,739 to Wright discloses an integrated fuel control system for a turbojet engine. Ambient air temperature, aircraft speed and altitude are all taken into account. The pilot controls the entire engine by a single manual control. In U.S. Pat. No. 3,091,080 to Crim et al, there is disclosed a fuel control system which combines the output of several engine condition sensors in order to derive a schedule for controlling the exhaust nozzle of an afterburning gas turbine engine.
In U.S. Pat. No. 3,460,554 to Johnson there is shown a control apparatus for positioning a shockwave in the diffuser associated with the compressor section of a jet engine. In U.S. Pat. No. 3,507,296 to Fix et al there is disclosed a fluid flow control apparatus which makes use of a bleed conduit to regulate the flow through a unit having a nozzle discharging a high pressure fluid through a supersonic diffuser section.
None of the above achieves what my invention discloses. It is an object of my invention to provide a positive flow of engine inlet air past the temperature sensing element of the fuel control unit. It is a further objective of the invention to provide a reliable flow of air without the use of mechanically moving parts.